1. Field of the Invention
The present invention relates generally to injection molding devices, and more particularly to hot manifolds suitable for use as part of the mold in such devices as a means to maintain the material in the runner channels of the mold in a liquid condition and at a substantially uniform temperature throughout the molding cycle of the injection molding device.
2. Summary of the Prior Art
The concept of a hot manifold is well known in the art of injection molding. Generally speaking, such a manifold represents an extension of the injection unit, adapted to maintain the melt in a liquid state, disposed within the mold, which is otherwise adapted to solidify the melt. In an injection molding device utilizing such a manifold, the melt proceeds from the extruder (or other melt source) through a sprue bushing (generally heated) from which it enters the manifold. Within the manifold, the melt passes through one or more runner channels exiting into one or more feeding channels, which convey it out of the manifold to the cavities (or cavity) of the mold through appropriate bushings or other connecting devices.
As was alluded to above, the purpose of manifolds of this type is to maintain the melt in the runner channels of the mold in a liquid condition at substantially the temperature at which it was extruded, uniformly, and throughout the molding cycle of the device. The accomplishment of this goal provides numerous advantages over the conventional practice wherein both the runners and the desired parts were solidified and ejected during each molding cycle. These advantages include, among others, more automatic operation; the elimination of the handling, regrinding and waste of the solidified runners; operation at lower pressures and temperatures; reduction of required press-plasticating capacity and shorter cycles because only the part must be molded, solidified and ejected during each cycle; and the reduction or elimination of various technical molding, gating, and ejection problems, primarily because the melt is delivered to the cavity at optimum flowability. There are also significant disadvantages and problems with presently available hot manifolds, however, which are primarily related to such factors as the means heretofore utilized to heat such manifolds, the complexity of the design and manufacture of such manifolds, and the inherent necessity of insulating such manifolds from the remainder of the mold in which they are contained.
Of particular importance is the fact that the hot manifold within the mold must be insulated from the cavity containing portion thereof. Air gaps have been found to be both economical and effective for this purpose in that they minimize the effect that the operating temperature of the manifold has upon the cavity portion of the mold and vice versa. The creation of such air gaps, on the other hand, requires the presence of supports between the manifold and the cavity containing portion of the mold which must be strong enough to withstand the clamping tonnage of the press and so designed that the manifold will not be damaged in machine operation. This means in many applications that either a large plurality of supports are needed or pressure pads of fairly large surface area must be utilized. These supports and pressure pads create localized heat sinks which prevent the achievement of a uniform temperature profile throughout the manifold.
When this factor is combined with the need to provide at least one melt channel between the hot manifold and the cooled cavity (ies), it will be understood that the task of heating the manifold uniformly is not an easy one. The prior art has attempted this in numerous ways with varying degrees of success. For example, commercially available cartridge type electrical resistance heaters have been located within the manifold near the runners. Similarly, tubular heaters have been embedded in the manifold block. In each of these alternatives, hot spots and an overall variation in the output temperature of the heater over its length have been noted. Attempts have been made to anticipate the location of such hot spots, the output temperature gradient and the nonuniformity caused by the heat sinking effects of the supports and related structures and to compensate therefor in the construction of the heaters, as by varying the coil density within such heaters along their length. Such attempts are marginally effective at best, and are totally impractical on a production scale due to the inherent variations found from one system to another. Alternatively, electrical resistance type heaters have been disposed within the melt stream contained in the runners either alone or in combination with the heaters located in the manifold block discussed above. Hot spots and a temperature gradient along the length of the heater plus external heat sinking are again significant problems however.
Further, in each of the above attempts at uniform manifold heating of the prior art, one must contend with the facts that each heater must be supplied with a controller and that the heater, controller, and associated electrical wire, must be located at a fairly inaccessible position within the mold. Accordingly, prior hot manifolds are not only complex to design and manufacture, and less than optimum in applied temperature uniformly, thereby causing melt degradation at worst and less than perfect operation at best, but also are inherently hard to mainftain and/or repair. In fact, if one of the above-referred-to resistance heaters fails, the manifold must generally be removed from the mold to replace it. This process is not only complex, expensive and time consuming in terms of disassembly and reassembly of the mold, but also extremely costly in terms of lost production caused by machine down time, wasted operator time and the like.
Additionally, despite the fact that the principals of heat pipe technology and design are, and have been for some years, well known (see for example, The Heat Pipe by G. Yale Eastman, Scientific American, May 1968, the disclosures of which are hereby incorporated herein by reference), they have been little used in the injection molding field. Thus, aside from the use of heat pipes to cool (or heat) localized portions of the core portion of the mold (U.S. Pat. No. 4,338,068 for example); the use of a heat pipe to heat the tip of a valve gate device (U.S. Pat. No. 4,125,352); and the use of heat pipes in an injection bushing (U.S. Pat. No. 4,034,952 for example), applicants are not aware of any relevant prior art in this field utilizing heat pipe technology. The reasons for this are not entirely clear, but may reside in the perceived complexity of design and manufacture arising from the fact that injection molding machine parts must be made of high strength steel in order to withstand the high operating pressures present in such devices. The iron in steel is, of course, not compatable with water, the normal operating fluid of a heat pipe (that is, the iron will tend to react with water at the temperatures and pressures of heat pipe operation to release the noncondensible gas hydrogen), thereby requiring not only the plating or coating of the inner walls of the heat pipe with a water compatable material such as copper or nickel, but also the provision of means such as a Monel (a nickel/copper/iron alloy) plate to allow the diffusion of any hydrogen gas created. This is particularly true in light of the ability of the art to "get by" with the use of more conventional electrical resistance heaters until the recent advent of a desire to injection mold plastics which exhibit extremely high thermo-sensitivities, that is plastics which can tolerate only very small temperature variations while passing through the machine in a molten state before they will degrade on the one hand or significantly lose flowability on the other.